AU2014218514B2

AU2014218514B2 - Wideband antenna system and method

Info

Publication number: AU2014218514B2
Application number: AU2014218514A
Authority: AU
Inventors: Ian Maxwell Davis; Christophe Jean-Marc Granet; John Seward Kot; Gregory Steven Pope
Original assignee: BAE Systems Australia Ltd
Current assignee: BAE Systems Australia Ltd
Priority date: 2013-02-21
Filing date: 2014-02-20
Publication date: 2018-02-08
Anticipated expiration: 2034-02-20
Also published as: AU2014218514A1; WO2014127420A1

Abstract

A wideband antenna including: a tapered elongated cavity defined by a first and second wall, a first proximal end of the cavity having a waveguide attached to an electromagnetic emission source; a second distal end of the cavity being electromagnetically transparent; the first wall having a sectional profile being axially tapered to a point; and the second wall being substantially monotonically increasing, to a first order, in radial diameter in section from the proximal to distal ends.

Description

FIELD OF THE INVENTION [0001] The present invention relates to the field of antenna devices and, in particular, to an antenna device having a wide frequency range of operation.

[0002] Antenna device of the present invention have a wide range of applications, including, but not limited to Radio Astronomy, Multi-band Satellite Communications (SATCOM) Systems, Signal Surveillance and Intelligence, Spectrum Surveillance, and Electronic Warfare.

BACKGROUND [0003] Any discussion of the background art throughout the specification should in no way be considered as an admission that such art is widely known or forms part of common general knowledge in the field.

[0004] Antenna devices efficiently couple energy to and from an oscillating electrical signal into a corresponding oscillating electromagnetic field normally initially operational within an associated waveguide. Ideally, the devices are operational over a wide range of frequencies of interest.

[0005] Examples of antenna devices (operational in a Multiband frequency domain) include those disclosed in: United States Patent 6,720,932 entitled “Multi-Frequency Antenna Feed”, United States Patent 8.089, 415 entitled “Multiband Radar Feed System and Method”, and United States Patent 6,982,679 entitled: “Coaxial Horn Antenna System”.

[0006] The desire for efficient transmission and reception of antenna signal over a wide frequency domain places restrictions on the particular antenna feed design. In particular, any form of non-linear frequency response for devices used in forming a wideband driving feed are likely to have detrimental effects on the performance of the antenna feed system. Further, the design is preferably operational over a very wide frequency range.

SUMMARY OF THE INVENTION [0007] It is an object of the invention, in its preferred form to provide an improved form of antenna design.

[0008] In accordance with a first aspect of the present invention, there is provided a wideband antenna including: a tapered elongated cavity defined by a first and second wall, a first proximal

2014218514 21 Dec 2017 end of the cavity having a waveguide attached to an electromagnetic emission source; a second distal end of the cavity being electromagnetically transparent; the first wall having a sectional profile being axially tapered to a point; and the second wall being substantially monotonically increasing, to a first order, in radial diameter in section from the proximal to distal ends.

[0009] The cavity can be substantially axially symmetric with the first wall forming a conical shape tapered to a point. The point can be spaced apart from the distal end of the cavity. In some embodiments, the second wall preferably can include a series of slots. The series of slots are preferably substantially axially symmetric.

[0010] In accordance with a further aspect of the present invention, there is provided a wideband antenna including: an elongated waveguide cavity having: an initial cylindrical waveguide having an initial annulus cross section at a first proximal end, with the thickness of the annulus (to a first order) monotonically expanding along a first axis.

[0011] In some embodiments, the diameter of the cross section of the inner surface of the waveguide axially reduces to a point. The point can be spaced apart from a distal end of the waveguide.

[0012] In accordance with a further aspect of the present invention, there is provided a wideband antenna as claimed in any one of the preceding claims wherein at least a section of said second wall is stepwise tapered.

[0013] In accordance with a further aspect of the present invention, there is provided a wideband antenna including a tapered elongated cavity defined by a first and second wall, a first proximal end of the cavity having a waveguide attachable to an electromagnetic emission source; a second distal end of the cavity being electromagnetically transparent; the first wall having a sectional profile being axially tapered to a point; and the second wall being substantially monotonically increasing, to a first order, in radial diameter in section from the proximal to distal ends.

[0014] In accordance with a further aspect of the present invention, there is provided a wideband antenna including an elongated waveguide cavity defined by an inner and outer surface of a cylindrical waveguide, which extends from a proximal end to a distal end, the waveguide cavity having an initial annulus cross section at the proximal end, with a thickness of the annulus (to a first order) monotonically expanding along a first axis towards the distal end, wherein the distal end of the cylindrical waveguide is electromagnetically transparent.

2014218514 21 Dec 2017

BRIEF DESCRIPTION OF THE DRAWINGS [0015] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0016] Fig. 1 illustrates the wideband coaxial launcher concept of the preferred embodiment;

[0017] Fig. 2 illustrates a perspective view of the feed portions of the wideband launcher;

[0018] Fig. 3 illustrates a sectional view through the feed portions of the wideband launcher;

[0019] Fig. 4 is a graph of the simulated measured frequency response of the wideband launcher;

[0020] Fig. 5 illustrates a schematic sectional view of the upper portion of a substantially symmetric ‘Bullet’ horn arrangement;

[0021] Fig. 6 illustrates a schematic sectional view of the Bullet Hom arrangement;

[0022] Fig. 7 illustrates a schematic sectional view of a first portion of a corrugated Bullet Hom arrangement;

[0023] Fig. 8 illustrates a schematic sectional view of a second operative portion of a corrugated Bullet Hom arrangement;

[0024] Fig. 9 illustrates a side perspective sectional view of an antenna design incorporating the principles of the preferred embodiment;

[0025] Fig. 10 illustrates a close up view of the feed portion of the antenna of Fig. 9;

[0026] Fig. 11 illustrates a further close up view of the feed portion of the antenna of Fig. 9;

[0027] Fig. 12 illustrates a higher power waveguide feed;

[0028] Fig. 13 illustrates further portions of the higher power feed; and [0029] Fig. 14 illustrates a shortened high power waveguide feed able to take probe inputs.

DETAILED DESCRIPTION [0030] The preferred embodiment provides for an efficient coupling over a wideband balanced excitation of a coaxial waveguide. The illustrated designs of the preferred embodiment are for operation in TEn mode and provide for a wideband transition from standard coaxial transmission line primarily to a TEn mode on a coaxial waveguide. The waveguide is fed to a coaxial hom antenna, herein after known as a ‘Bullet Hom’ antenna device.

2014218514 21 Dec 2017 [0031] The waveguide can then be used to feed various antenna devices, for example, a coaxial horn antenna.

[0032] For typical coaxial waveguide dimensions used in the preferred embodiment, the mode spectrum of the waveguide, in order of increasing cut-off frequency is: TEM, TEn, TE₂i, ΤΕ31, .... TE_mi. By exciting the coaxial waveguide with a pair of balanced probes, the TE_mi modes with m even are nulled-out, as is the TEM mode.

[0033] This leaves the TEn mode as the first significant excited mode, with the TE31 mode as the next significant higher-order mode. To a good approximation, the cut-off frequency of the TE₃i mode is three-times the cut-off frequency of the TEn mode. This fixes the practical upper limit for the operating frequency bandwidth of such a feed as 3:1. However, practical considerations mean that the waveguide should be operated somewhat above the TEn cut-off frequency, so a more realistic limit for the frequency bandwidth is probably closer to 2.5:1.

[0034] The spectrum of the coaxial waveguide can be modified by including ridges or corrugations into the waveguide, and this approach has the potential to extend this limit.

[0035] A significant practical limit of this type of transition is the need for a balanced feed to the two probes, i.e. they need to be driven with signals of equal amplitude that are 180° out of phase. Generating this equal amplitude split with a 180° phase shift is possible with a component such as a hybrid junction or a Baiun, but these components are also frequencydependent, and their bandwidth limits tend to limit the overall performance of the wideband transition.

[0036] The preferred embodiment takes advantage of the topology of the coaxial waveguide to allow connection via the inner and outer conductors, to allow in-phase excitation of the junction. The resulting in-phase power divider can be made to operate over a very wide bandwidth, so that the frequency dependence of the wideband transition is not limited by the performance of the power splitter.

[0037] A wideband coaxial launcher concept 1 of the preferred embodiment is illustrated initially in Fig. 1. In this case, a pair of coaxial concentric waveguides 2, 3 is provided. Waveguides 2, 3 extend circumferentially and coaxially about a central axis with waveguide 2 being disposed within waveguide 3, which has a larger radius than waveguide 2. The region between waveguides 2, 3 defines an annulus shaped cavity centred about the central axis and extending along the axis. The annulus cavity is substantially axially symmetric about the central axis.

2014218514 21 Dec 2017 [0038] Two probes 5, 6 located within the annulus at substantially diametrically opposed positions are driven from opposite ends with respective in-phase signals 8, 9 to generate a balanced excitation. In this way, the need for 180° phase shift circuitry is removed. If the probes 5, 6 are matched using a system coaxial impedance of 100Ω, the two probes can be connected in parallel to a standard 50Ω coaxial line 10 using a tee-junction 11 to generate the inphase signals. Because the match of the tee-junction depends only on characteristic impedance of the transmission lines, it is inherently frequency-independent, allowing for a wideband driving signal.

[0039] The arrangement of Fig. 1 deals only with a single polarization, ffowever, the orthogonal polarization can be generated using a set of orthogonal probes. An advantage over the known circular ridged waveguide wideband orthomode transducer (OMT) is that it is simple to locate the probes for both polarizations in a common plane. This can also apply to a ridged coaxial OMT.

[0040] One possible implementation of a wideband junction arrangement 20 is shown in Fig. 2. The arrangement 20 utilises a symmetrical double-tapered probe structure 21, 22 that can be fed either via a transmission line 23 connected to the inner conductor or a transmission line 24 connected to the outer conductor of the large coaxial waveguide. Each probe has a structure that is widest at a point intermediate the inner and outer conductors and which tapers in width towards each conductor, as shown in Fig. 2. The probes 21, 22 are connected to equal lengths of 100Ω coaxial transmission line 23, 24, and then via a tee-junction 25 to a 50Ω coaxial line 26 and input connector.

[0041] In the embodiment illustrated in Fig. 2, probe 22 is driven by transmission line 24 from a position external to the outer conductor of the coaxial waveguide and probe 21 is driven by transmission line 23 from a position internal to the inner wall of the coaxial waveguide.

[0042] Impedance matching can be done using three structures: the coaxial cavity 28 behind the probes, the shape of the probes 21, 22 themselves, and a pair of impedance matching stubs 29, 30 placed in front of the probes at a predetermined distance from the respective probes within the annulus. Although a pair of matching stubs 29, 30 is shown in Fig. 2, alternative matching mechanisms can be used, like ridges, irises, steps, etc.

[0043] Fig. 3 illustrates a sectional view through the arrangement 20 of Fig. 2. As the probes 21, 22 are fed from different ends and are located in a coaxial structure, they are only approximately symmetrical in structure and so the two probes will be slightly different to achieve a balanced feed.

2014218514 21 Dec 2017 [0044] An example coaxial feed launcher of the arrangement of Fig. 3 was analysed using the software package CST Microwave Studio. The initial, non optimised results for S- parameters are shown in Fig. 4. A first curve 41 shows SI 1 at the 50Ω input, and the second curve 42 shows S31 which is the coupling from the 50Ω input to the TE11 mode at the output. The target frequency band was 1 to 2 GHz, but a slight overall frequency shift has occurred, giving an operating frequency band of approximately 1.1 to 2.3 GHz.

INTERFACE TO A WIDEBAND COAXIAL HORN ANTENNA [0045] In the design of the preferred embodiment, the primary function of the wideband coaxial junction is to feed a mated wideband coaxial horn antenna. In the preferred embodiment the coaxial horn includes a profiled surface, hereinafter referred to as a “Bullet Hom”, which interfaces directly with the wideband coaxial junction.

[0046] Given the coaxial waveguide input parameters of the wideband coaxial junction, a set of parameterized profile curves (like splines for example) can be defined to generate the Bullet Hom shape. Two profiles are required, an inner profile and an outer profile. An example resultant design of the Bullet Hom structure can be as illustrated in Fig. 5, which illustrates a sectional view through an upper portion of a substantially axially symmetric Bullet Hom 50. The Bullet Hom arrangement includes two profiled surfaces 51, 52 defined by a number of spline-nodes e.g. 53 that are used as parameters to define the surface geometry. In the case where spline curves are used to define the profiles, the spline nodes are the parameters that can be utilized to optimize the overall performance of the combined wideband coaxial junction and Bullet Hom. Fig. 6 illustrates a sectional profile view of one form of Hom geometry, illustrating its substantially symmetric nature.

[0047] The optimization process can take into account a set of user defined performance requirements such as the overall input return loss, gain, cross-polarization maximum and sidelobe levels. An optimization procedure adjusts the inner and outer profiles and “shapes” the Bullet Hom profile to meet or come as close as possible to a desired performance targets.

[0048] The Bullet Hom geometry can be, at present, either smooth-walled or corrugated. Whilst Fig. 5 and Fig. 6 illustrate a smooth walled design, Fig. 7 and Fig. 8 illustrate a corrugated wall design. Fig. 7 illustrates a first sectional view 70 of the top portion of an axially symmetric Bullet Hom design. The top portion initially includes tapered spline profiled surfaces 72, 73, which then feed out to a corrugated hom profile end. The corrugations provide for low cross polarisation of the antenna system. Fig. 8 illustrates the overall geometry of the corrugated hom arrangement.

2014218514 21 Dec 2017

MECHANICAL DESIGN CONCEPTS [0049] A mechanical design for a wideband coaxial launcher and associated Bullet Hom was investigated from a manufacturing perspective. Fig. 9 to Fig. 11 illustrate sectional views through one investigated design. Turning initially to Fig. 9, in this design, a corrugated horn arrangement 90 is illustrated having a front end 91 with a corrugated and radially expanding outer profded surface and a back end 92 having a radially tapered inner profded surface 93. The coaxial feed in is provided in back section 94. The front end 91 and back end 92 are connected at a point where the inner surface 93, which is conical in shape, tapers to a point. In one embodiment, front end 91 includes a series of corrugated slots along the outer profded surface.

[0050] Fig. 10 illustrates an enlarged view of the back portion of the Bullet Hom antenna, showing the stepwise profded surfaces 92, 93. In preferred embodiments, the surfaces 92, 93 are stepwise tapered so as to include a series of distinct but interconnected taper levels. Similarly, surfaces 72, 73 of Fig. 7 can include stepwise profded sections. In various embodiments, the profdes surfaces can include various degrees of smoothing, which define the size of each taper level. As such, in some embodiments, the outer profiled surface increases substantially monotonically in radial diameter from a proximal end adjacent the back end 92 to a distal end adjacent front end 91. At the distal end, the cavity is electromagnetically transparent.

[0051] The coaxial feed in is provided by means of coaxial cable 98 which is split into two cables 96, 97 which deliver signals to the probes phased appropriately to excite the TEn mode. The tapering of inner profiled surface 93 corresponds to a tapering of the inner surface of the annulus between conductors. Therefore, an inner core of the annulus tapers down to a point beyond which the region within waveguide 3 is circular in radial cross section.

[0052] Fig. 11 illustrates the feed in portion 94 of Fig. 10 in more detail. The coaxial cable 98 is split with two equal lengths 96, 97 being fed to corresponding probes 101, 102. The coaxial cable 97 passes through a core cavity 106 and attaches to the probe 102. Cavity tuning is provided by tuning stubs 103, 104.

[0053] The arrangement of Fig. 9 to Fig. 11 provides a mechanically sound and inexpensive antenna feed arrangement.

USE OF ALTERNATIVE FEED STRUCTURES FOR HIGH-POWER AND LOWLOSS OPERATION [0054] The foregoing arrangement will only apply up to a particular power limit. These arrangements use traditional coaxial transmission lines to drive the probes 101, 102 that are used

2014218514 21 Dec 2017 to excite the large coaxial waveguide. Coaxial lines may not be ideal for some applications, due to limited power-handling or high loss. One alternative approach would be the direct replacement of the coaxial transmission lines with an alternative TEM-mode transmission line, such as slab-line.

[0055] When dealing with high power environments, a preferred approach is to utilise a wideband waveguide structure, such as a ridged rectangular waveguide. This second approach would be facilitated by the replacement of the probe feeds by coupling slots.

[0056] Fig. 12 illustrates one form of Electromagnetic (EM) model of a coaxial OMT 120 with double ridged waveguide ports including ports 121 to 124. The double ridged waveguide ports can be fed with a double ridged waveguide network containing wideband E-plane Tee junctions. Such an arrangement 130 is illustrated in Fig. 13, wherein the coaxial OMT 120 is fed by double ridged waveguide ports 121 to 124 which are in turn coupled to E-plane Tee junctions 131, 132. Tee junctions 131, 132 can be respectively configured to deliver input electromagnetic signals in orthogonal polarisations. Tee junctions 131, 132 split the power from each of the orthogonal polarised signals to deliver a first polarisation along waveguide ports 121 and 122, and a second polarisation along waveguide ports 123 and 124. As illustrated in Fig. 13, ports 121 and 122 are diametrically opposed to each other. Ports 123 and 124 are similarly diametrically opposed to each other. All four ports are arranged around the circumference of a cylindrical waveguide. The cylindrical waveguide is able to be interconnected with the feed portion of the Bullet Hom antenna described above.

[0057] In an alternative medium to high power arrangement 140, as illustrated in Fig. 14, the double ridged waveguide output ports are transformed into standard coaxial ports 141-144. The coaxial ports are on opposing sides which results in each pair of ports being in anti-phase if fed by the same simple coaxial splitter as proposed for the wideband coaxial launcher.

[0058] It can be seen that the described arrangements provide for the utilisation of symmetrical probes fed from opposite ends, which gives the desired balanced feed, and provides for a practical implementation concept, for example, for a feed for a wideband coaxial horn antenna. A similar concept can be realised using ridged waveguides coupled via slots, for example, in high-power applications.

[0059] The wideband coaxial junction and Bullet Hom design can be used together to achieve user defined wideband performance in terms of return loss, radiation pattern and gain.

2014218514 21 Dec 2017

INTERPRETATION [0060] Reference throughout this specification to “one embodiment”, “some embodiments” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment”, “in some embodiments” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

[0061] As used herein, unless otherwise specified the use of the ordinal adjectives first, second, third, etc., to describe a common object, merely indicate that different instances of like objects are being referred to, and are not intended to imply that the objects so described must be in a given sequence, either temporally, spatially, in ranking, or in any other manner.

[0062] In the claims below and the description herein, any one of the terms comprising, comprised of or which comprises is an open term that means including at least the elements/features that follow, but not excluding others. Thus, the term comprising, when used in the claims, should not be interpreted as being limitative to the means or elements or steps listed thereafter. For example, the scope of the expression a device comprising A and B should not be limited to devices consisting only of elements A and B. Any one of the terms including or which includes or that includes as used herein is also an open term that also means including at least the elements/features that follow the term, but not excluding others. Thus, including is synonymous with and means comprising.

[0063] As used herein, the term “exemplary” is used in the sense of providing examples, as opposed to indicating quality. That is, an “exemplary embodiment” is an embodiment provided as an example, as opposed to necessarily being an embodiment of exemplary quality.

[0064] It should be appreciated that in the above description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this

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Detailed Description, with each claim standing on its own as a separate embodiment of this invention.

[0065] Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those skilled in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

[0066] Furthermore, some of the embodiments are described herein as a method or combination of elements of a method that can be implemented by a processor of a computer system or by other means of carrying out the function. Thus, a processor with the necessary instructions for carrying out such a method or element of a method forms a means for carrying out the method or element of a method. Furthermore, an element described herein of an apparatus embodiment is an example of a means for carrying out the function performed by the element for the purpose of carrying out the invention.

[0067] In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

[0068] Similarly, it is to be noticed that the term coupled, when used in the claims, should not be interpreted as being limited to direct connections only. The terms coupled and connected, along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Thus, the scope of the expression a device A coupled to a device B should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. Coupled may mean that two or more elements are either in direct physical or electrical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other.

[0069] Thus, while there has been described what are believed to be the preferred embodiments of the invention, those skilled in the art will recognize that other and further modifications may be made thereto without departing from the spirit of the invention, and it is intended to claim all such changes and modifications as falling within the scope of the invention. For example, any formulas given above are merely representative of procedures that may be used. Functionality may be added or deleted from the block diagrams and operations may be

2014218514 21 Dec 2017 interchanged among functional blocks. Steps may be added or deleted to methods described within the scope of the present invention.

2014218514 21 Dec 2017

Claims

CLAIMS:

1. A wideband antenna including:

a tapered elongated cavity defined by a first and second wall, a first proximal end of the cavity having a waveguide attachable to an electromagnetic emission source;

a second distal end of the cavity being electromagnetically transparent;

the first wall having a sectional profile being axially tapered to a point; and the second wall being substantially monotonically increasing, to a first order, in radial diameter in section from the proximal to distal ends.
2. A wideband antenna as claimed in claim 1 wherein the cavity is substantially axially symmetric with the first wall forming a conical shape tapered to a point.
3. A wideband antenna as claimed in claim 2 wherein the point is spaced apart from the distal end of the cavity.
4. A wideband antenna as claimed in any previous claim wherein said second wall includes a series of slots.
5. A wideband antenna as claimed in claim 4 wherein said series of slots are substantially axially symmetric.
6. A wideband antenna as claimed in any one of the preceding claims wherein at least a section of said second wall is stepwise tapered.
7. A wideband antenna including an elongated waveguide cavity defined by an inner and outer surface of a cylindrical waveguide, which extends from a proximal end to a distal end, the waveguide cavity having an initial annulus cross section at the proximal end, with a thickness of

2014218514 21 Dec 2017 the annulus (to a first order) monotonically expanding along a first axis towards the distal end, wherein the distal end of the cylindrical waveguide is electromagnetically transparent.
8. A wideband antenna as claimed in claim 7 wherein the diameter of the cross section of the inner surface of the waveguide axially reduces to a point.
9. A wideband antenna as claimed in claim 7 wherein the point is spaced apart from a distal end of the waveguide.

WO 2014/127420

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Figure 1-1: Wideband coax launcher concept

FIG. 1

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FIG. 4

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x. imm;

FIG. 6

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0.7-1,7GHz Hom: Initial Geometry

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FIG. 10

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FIG. 14

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